Magnesium oxide is an inorganic compound with good thermal conductivity, heat resistance, and other properties, and used as a thermally conductive filler to form resin compositions with higher thermal conductivity in combination with a variety of resins. On the other hand, magnesium oxide, which has a relatively high ability to hydrate, can easily absorb water to expand in volume and suffer from cracking and other defects. This has been a barrier to the commercialization of magnesium oxide as a thermally conductive filler. Therefore, a technique to improve water resistance has been demanded for the use of magnesium oxide as a thermally conductive filler.
For example, there is known a technique of improving water resistance by a process that includes coating the surface of a magnesium oxide powder with silica or other materials, then baking the powder to form a layer of a complex oxide such as forsterite (Mg2SiO4), and further performing a phosphoric acid treatment to form a magnesium phosphate compound layer (see, for example, Patent Literature 1). There is also known a technique in which the surface after the phosphoric acid treatment is further treated with an organosilicate, so that an organosilicate coating layer is formed to improve water resistance and prevent alkali leaching (see, for example, Patent Literature 2).